Process for producing cast steel billet or steel ingot of titanium-added case hardening steel

ABSTRACT

Disclosed is a production process that can suppress the formation of large-diameter TiN or N-rich TiCN inclusions which adversely affect machinability and consequently can produce a cast steel billet or steel ingot or a steel material of a Ti-added case hardening steel having excellent machinability. The production process comprises the step of casting a steel comprising by weight carbon (C): 0.10 to 0.30%, titanium (Ti): 0.05 to 0.20%, and nitrogen (N): not more than 0.0100% into a round cast steel billet or steel ingot having a diameter of not more than 23 cm or, in the alternative, a rectangular cast steel billet or steel ingot having a short side length of not more than 23 cm.

CROSS-REFERENCE OF RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 118602/2006, filed on Apr. 26, 2006, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a process for producing a cast steel billet or steel ingot of a titanium (Ti)-added case hardening steel as a steel for machine construction for use in automobile components and components of other various industrial machines or apparatuses.

2. Background Art

Case hardening steels have hitherto been generally brought to a predetermined product shape, for example, by hot or cold working or machining followed by carburization (900° C. or above) quenching and tempering treatment. However, some combination of a structure before carburization and carburization conditions causes grain coarsening upon the carburization, leading to deteriorated fatigue strength, static strength or other properties of steel products. In case hardening steels, precipitates of AlN or NbCN are utilized as pinning particles from the viewpoint of preventing this unfavorable phenomenon. To this end, composition adjustment of steel products is sometimes carried out, for example, for aluminum (Al), niobium (Nb), and nitrogen (N). Titanium-added case hardening steels are also steel products to which titanium has been added for grain coarsening preventive purposes. Such titanium-added case hardening steels, however, suffer from a problem that, due to the presence of a large amount of hard TiN or N-rich TiCN, subsequent machining is difficult. This tendency is particularly significant for titanium-added case hardening steels melted in a mass-production furnace.

The applicant of the present invention has proposed an invention relating to a titanium-added high-strength steel as a titanium-added steel in which the formation of TiN or N-rich TiCN, which adversely affects fatigue properties of titanium-added steels for machine construction, has been suppressed, the (area max)^(1/2) (hereinafter referred to as “√ (area max)”) as the maximum size of TiN or TiCN crystallized in the steel being not more than 80 μm in a measurement area of 30,000 mm² estimated by an extreme value statistical method (see, for example, Japanese Patent Laid-Open No. 143550/2004). This titanium-added high-strength steel has improved fatigue strength, but on the other hand, the machinability is not always excellent.

SUMMARY OF THE INVENTION

The present inventors have found that, in producing a cast steel billet or steel ingot of a titanium-added case hardening steel, defining the size of the cast steel billet or steel ingot of a titanium-added case hardening steel within a predetermined range in a specific steel compositional range can suppress the formation of large-diameter TiN or N-rich TiCN inclusions, which adversely affect machinability, and consequently can produce a cast steel billet, a steel ingot or a steel product of a titanium-added case hardening steel having excellent machinability. Specifically, the present inventors have found that the above advantage can be provided by regulating the diameter in the case of a round cast steel billet or steel ingot and the short side length in the case of a rectangular cast steel billet or steel ingot to not more than 23 cm.

Accordingly, an object of the present invention is to provide a production process that can suppress the formation of large-diameter TiN or N-rich TiCN inclusions, which adversely affect machinability, and consequently can produce a cast steel billet, a steel ingot or a steel product of a titanium-added case hardening steel having excellent machinability.

According to the present invention, there is provided a process for producing a cast steel billet or steel ingot of a titanium (Ti)-added case hardening steel, said process comprising the steps of:

melting a steel comprising by weight carbon (C): 0.10 to 0.30%, titanium (Ti): 0.05 to 0.20%, and nitrogen (N): not more than 0.0100%; and

casting the molten steel into a round cast steel billet or steel ingot having a diameter of not more than 23 cm.

According to another aspect of the present invention, there is provided a process for producing a cast steel billet or steel ingot of a titanium (Ti)-added case hardening steel, said process comprising the steps of:

melting a steel comprising by weight carbon (C): 0.10 to 0.30%, titanium (Ti): 0.05 to 0.20%, and nitrogen (N): not more than 0.0100%; and

casting the molten steel into a rectangular cast steel billet or steel ingot having a short side length of not more than 23 cm.

In a preferred embodiment of the present invention, the steel comprises by weight

carbon (C): 0.10 to 0.30%,

silicon (Si): 0.03 to 1.00%,

manganese (Mn): 0.20 to 1.00%,

phosphorus (P): not more than 0.025%,

sulfur (S): 0.001 to 0.030%,

nitrogen (N): not more than 0.0100%,

titanium (Ti): 0.05 to 0.20%,

aluminum (Al): 0.003 to 0.050%,

oxygen (O): not more than 0.0030%,

one or more of chromium (Cr): 0.15 to 2.00%, nickel (Ni): 0.10 to 2.00%, molybdenum (Mo) 0.03 to 0.30%, boron (B): 0.0001 to 0.0020%, vanadium (V): 0.01 to 0.20%, and niobium (Nb): 0.01 to 0.15%, and

the balance iron (Fe) and unavoidable impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between cast steel billet and steel ingot sizes and the √ (area max) value as the maximum size of TiN or TiCN;

FIG. 2 is a graph showing the relationship between the √ (area max) value as the maximum size of TiN or TiCN and the abrasion loss of chips in a carbide turning test; and

FIG. 3 is a graph showing the relationship between the √ (area max) value as the maximum size of TiN or TiCN and the number of holes formed by drilling.

DETAILED DESCRIPTION OF THE INVENTION

In the production process of a cast steel billet or steel ingot of a titanium-added case hardening steel according to the present invention, a steel comprising by weight carbon (C): 0.10 to 0.30%, titanium (Ti): 0.05 to 0.20% and nitrogen (N): not more than 0.0100% is first provided. The reasons for the limitation of the contents of these individual constituents will be described. In the following description, “%” is by weight.

Carbon (C) is an element that affects hardenability, forgeability, and machinability of the material in its core part. When the carbon content is less than 0.10%, the hardness of the core part is unsatisfactory and, consequently, the strength is lowered. Accordingly, the carbon content should be not less than 0.10%. When the carbon content exceeds 0.30%, however, the hardness of the material is increased and, consequently, workability such as machinability and forgeability is hindered. For the above reason, the content of carbon in the steel according to the present invention is limited to 0.10 to 0.30%.

Titanium (Ti) is an element useful for fining grains in carburization. When the titanium content is less than 0.05%, the effect of fining grains in the carburization cannot be attained. Accordingly, the titanium content should be not less than 0.05%. When the titanium content exceeds 0.20%, however, the formation of TiN or TiCN hinders machinability. For the above reason, the content of titanium in the steel used in the present invention is limited to 0.05 to 0.20%, preferably 0.05 to 0.15%.

Nitrogen (N), as described above, when the titanium content exceeds 0.20%, is reacted with titanium to form TiN or TiCN which hinders machinability. Accordingly, the nitrogen content is limited to not more than 0.0100%, preferably not more than 0.0080%, typically 20 ppm to 0.0080%.

In a preferred embodiment of the present invention, the steel used in the present invention may preferably contain silicon (Si) of 0.03 to 1.00%, more preferably 0.05 to 0.40%. Silicon is an element useful for deoxidation, and the silicon content of not less than 0.03% enhances this effect. When the silicon content exceeds 1.00%, however, the hardness of the material is increased and, consequently, workability such as machinability and forgeability is hindered.

In a preferred embodiment of the present invention, the steel used in the present invention may preferably contain manganese (Mn) of 0.20 to 1.00%, more preferably 0.25 to 0.70%. Manganese is an element useful for ensuring hardenability, and the manganese content of not less than 0.20% enhances this effect. When the manganese content exceeds 1.00%, however, the hardness of the material is increased, workability such as machinability and forgeability is hindered.

In a preferred embodiment of the present invention, the steel used in the present invention may preferably contain sulfur (S) of 0.001 to 0.030%, more preferably 0.003 to 0.020%. Sulfur is an element useful for ensuring the machinability, and the sulfur content of not less than 0.001% enhances this effect. When the sulfur content exceeds 0.030%, however, the cold workability is hindered and the fatigue strength is deteriorated.

In a preferred embodiment of the present invention, the steel used in the present invention may preferably contain aluminum (Al) of 0.003 to 0.050%. Aluminum is an element useful for deoxidation in the steelmaking, and the aluminum content of not less than 0.003% enhances deoxidation. When the aluminum content exceeds 0.050%, however, nonmetallic inclusions are produced, resulting in lowered fatigue strength.

In a preferred embodiment of the present invention, the steel used in the present invention may preferably contain phosphorus (P) of not more than 0.025%. This is because phosphorus is an impurity that renders grain boundaries of carburized components brittle.

In a preferred embodiment of the present invention, the steel used in the present invention may preferably contain oxygen (O) of not more than 0.0030%, because oxygen produces various nonmetallic inclusions which render grain boundaries of carburized components brittle.

In a preferred embodiment of the present invention, the steel used in the present invention may preferably contains one or more of chromium (Cr): 0.15 to 2.00%, nickel (Ni): 0.10 to 2.00%, molybdenum (Mo): 0.03 to 0.30%, boron (B): 0.0001 to 0.0020%, vanadium (V): 0.01 to 0.20% and niobium (Nb): 0.01 to 0.15%. Chromium (Cr) is an element useful for ensuring hardenability and improving toughness, and the chromium content of not less than 0.15% enhances this effect. When the chromium content exceeds 2.00%, however, the hardness of the material is increased and, consequently, workability such as machinability and forgeability is hindered. Nickel (Ni) is an element useful for improving toughness, and the nickel content of not less than 0.10% enhances this effect. When the nickel content exceeds 2.00%, however, the hardness of the material is increased and, consequently, workability such as machinability and forgeability is hindered, resulting in increased cost. Molybdenum (Mo) is an element useful for improving toughness, and the molybdenum content of not less than 0.03% enhances this effect. When the molybdenum content exceeds 0.30%, however, the hardness of the material is increased and, consequently, workability such as machinability and forgeability is hindered, resulting in increased cost. Boron (B) is an element useful for improving hardenability, and the boron content of not less than 0.0001% enhances this effect. When the boron content exceeds 0.0020%, however this effect is saturated. Vanadium (V) is an element useful for fining grains to improve toughness, and the vanadium content of not less than 0.01% enhances this effect. When the vanadium content exceeds 0.20%, however, the effect of refining grains is saturated, resulting in increased cost. Niobium (Nb) is an element useful for refining grains to improve toughness, and the niobium content of not less than 0.01% enhances this effect. When the niobium content exceeds 0.15%, however, the effect of refining grains is saturated, resulting in increased cost.

In the production process according to the present invention, a steel having the above chemical composition is melted, and the melted steel is cast into a round cast steel billet or round steel ingot having a diameter of not more than 23 cm or, in the alternative, a rectangular cast steel billet or rectangular steel ingot having a short side length of not more than 23 cm. When the diameter of the round cast steel billet or round steel ingot or the short side length of the rectangular cast steel billet or rectangular steel ingot exceeds 23 cm, the size of TiN or TiCN inclusions is increased and, consequently, the machinability is adversely affected. Casting into a cast steel billet or steel ingot having a diameter or short side length of not more than 23 cm can reduce the size of TiN and TiCN inclusions and can reduce the adverse effect on machinability. For the above reason, the diameter of the round cast steel billet or round steel ingot or the short side length of the rectangular cast steel billet or rectangular steel ingot is limited to not more than 23 cm.

In a preferred embodiment of the present invention, in the cast steel billet or steel ingot thus produced, the maximum size: (area max)¹¹² (hereinafter referred to as “√ (area max)”) of TiN or TiCN crystallized in the steel in a measurement area of 30,000 mm² estimated by an extreme value statistical method may be not more than 50 μm, more preferably not more than 30 μm. When the √ (area max) value is in this range, excellent machinability can be realized.

The √ (area max) may be calculated based on the method described in Murakami Yukitaka, “Kinzoku Hiro Bisho Kekkan to Kaizaibutu no Eikyo (Influence of Metal Fatigue Microdefects and Inclusions),” Yokendou Co., Ltd, Mar. 8, 1993, pp. 233 to 241. The description of this document is referred to herein, and the entire contents thereof are incorporated herein by reference.

In a preferred embodiment of the present invention, the √ (area max) value is specifically calculated by adopting conditions of reference microscopic examination area S_(o): 100 mm², number of microscopic visual fields n: 30, and prediction area S: 30,000 mm² according to the following procedure. At the outset, search is made for the largest inclusion in one microscopic sample having a size of 100 mm². The size of the maximum inclusion is measured, and the √ (area max) value is calculated according to the equation √ (area max)=√ (major axis×minor axis). This measurement is repeatedly carried out for 30 visual fields. The data thus obtained are rearranged in the order from small to large √ (area max) values, and the rearranged data are plotted as √ (area max), _(j), wherein j=1 to n, on an extreme value statistical graph. In this case, in the extreme value statistical graph, the abscissa represents √ (area max), and one of ordinates represents a cumulative distribution function: Fj=j/(n+1) while the other ordinate represents a normalization variable Yj=−ln (−ln(−ln(Fj)). Finally, a largest inclusion distribution straight line is determined by a least square method, and the inclusion diameter is estimated from the normalization variable in the prediction area S.

Specifically, in a cast steel billet or steel ingot of a titanium-added case hardening steel or its steel product produced by the process according to the present invention, the √ (area max) value as the maximum size of TiN or TiCN inclusions which adversely affect machinability can be brought to not more than 50 μm by limiting the size of the cast steel billet or steel ingot in the production of a steel product to not more than 23 cm in terms of diameter for a round type and short side length for rectangular type. As a result, according to the present invention, a cast steel billet or steel ingot or a steel product of a titanium-added case hardening steel having excellent machinability can be provided. A titanium-added case hardening steel product produced by hot rolling or hot forging the cast steel billet or steel ingot according to the present invention has unprecedented excellent effect.

EXAMPLES

The following Examples further illustrate the present invention. It should be noted that the present invention is not limited to these Examples.

Steels comprising constituents shown in Tables 1 and 2 were produced in a 90-ton electric furnace by a melt process, or were produced in a 1-ton vacuum melting furnace (1t-VIM furnace) by melting and were cast to obtain cast steel billets and steel ingots having various sizes. Specifically, for steels produced in the electric furnace, cast steel billets having a size of 38 cm in short side length×49 cm in long side length were produced. On the other hand, for steels produced in the vacuum melting furnace, steel ingots having a size of 16 cm in short side length×18 cm in long side length, and round steel ingots having a diameter of 30 cm and round steel ingots having a diameter of 19 cm were produced. These cast steel billets and steel ingots were heated to a temperature of 1230° C. or above followed by hot forging into steel bars having a size of 60 mmφ and a size of 32 mmφ. The steel bars were normalized at 900° C. for one hr, and test pieces were then prepared from the normalized steel bars. The chemical composition and the applied melting furnace for titanium-added case hardening steel comprising constituents specified in claim in the present invention as titanium-added steels (chemical composition falling within the scope of claim) in the classification are shown in Nos. 1 to 20 in Table 1. In Table 1, the underlined values for chromium, nickel and molybdenum are values for unavoidably contained impurities. Steels of which the titanium content is below the lower limit of the titanium content range of the present invention are shown as comparative steels in Nos. 21 to 28 as the titanium-free steel in the classification in Table 2. Steels in which the titanium content is outside the scope of the titanium content range of the present invention or in which, although the titanium content satisfies the content range of the present invention, other chemical constituents do not satisfy the content range of the present invention, are shown in Nos. 29 to 36 as titanium-added steels (chemical composition outside the scope of claim) in the classification. In these steels, values in the constituents outside the scope of the present invention are underlined. The chemical compositions are also be shown, and, further, the type of the applied melting furnace is shown as an electric furnace or 1t-VIM. TABLE 1 (Mass %) Melting No. Classification C Si Mn P S Cr Ni Al Mo Ti N O Others furnace 1 Ti-added steel 0.14 0.10 0.28 0.023 0.008 0.81 1.01 0.021 0.10 0.121 0.0071 0.0009 Electric (chemical furnace 2 composition 0.15 0.13 0.38 0.020 0.010 0.84 0.99 0.025 0.12 0.114 0.0074 0.0009 1t-VIM 3 falling within 0.15 0.09 0.32 0.021 0.012 0.78 0.98 0.028 0.12 0.118 0.0062 0.0012 1t-VIM 4 scope of claim) 0.15 0.08 0.32 0.021 0.012 0.78 1.03 0.030 0.11 0.125 0.0068 0.0025 1t-VIM 5 0.17 0.09 0.40 0.018 0.029 1.00 0.08 0.013 0.02 0.142 0.0063 0.0010 B: 0.0019 Electric furnace 6 0.18 0.08 0.42 0.015 0.025 1.00 0.05 0.017 0.02 0.145 0.0062 0.0009 B: 0.0008 1t-VIM 7 0.17 0.09 0.41 0.017 0.024 1.00 0.03 0.022 0.01 0.140 0.0063 0.0013 B: 0.0011 1t-VIM 8 0.18 0.08 0.40 0.015 0.024 1.00 0.04 0.025 0.01 0.152 0.0059 0.0015 B: 0.0018 1t-VIM 9 0.19 0.09 0.64 0.014 0.016 1.34 0.09 0.024 0.02 0.053 0.0045 0.0009 Nb: 0.03 Electric furnace 10 0.20 0.09 0.68 0.013 0.014 1.34 0.04 0.023 0.02 0.065 0.0042 0.0007 Nb: 0.02 1t-VIM 11 0.19 0.10 0.65 0.014 0.016 1.34 0.05 0.020 0.02 0.067 0.0041 0.0013 Nb: 0.03 1t-VIM 12 0.18 0.09 0.68 0.016 0.018 1.34 0.04 0.028 0.02 0.054 0.0049 0.0012 Nb: 0.05 1t-VIM 13 0.24 0.08 0.59 0.021 0.019 1.78 0.08 0.018 0.14 0.119 0.0071 0.0007 Electric furnace 14 0.25 0.07 0.54 0.018 0.019 1.78 0.05 0.019 0.15 0.128 0.0075 0.0009 1t-VIM 15 0.22 0.06 0.49 0.017 0.019 1.78 0.06 0.022 0.15 0.131 0.0061 0.0016 1t-VIM 16 0.23 0.07 0.48 0.019 0.019 1.78 0.07 0.024 0.16 0.132 0.0079 0.0019 1t-VIM 17 0.26 0.12 0.50 0.014 0.007 0.59 0.55 0.024 0.02 0.113 0.0075 0.0011 B: 0.0010 Electric furnace 18 0.27 0.11 0.54 0.012 0.006 0.62 0.60 0.020 0.02 0.119 0.0063 0.0012 B: 0.0016 1t-VIM 19 0.28 0.10 0.47 0.016 0.003 0.58 0.45 0.018 0.02 0.124 0.0073 0.0014 B: 0.0019 1t-VIM 20 0.25 0.10 0.45 0.019 0.004 0.12 0.54 0.024 0.02 0.129 0.0068 0.0013 B: 0.0015 1t-VIM * In Table 1, the values in the underlined part are values regarding impurities.

TABLE 2 (Mass %) Melting No. Classification C Si Mn P S Cr Ni Al Mo Ti N O Others furnace 21 Ti-free steels 0.19 0.24 0.78 0.021 0.013 1.10 0.09 0.023 0.02 0.001 0.0138 0.0007 Electric (comparative furnace 22 steels) 0.20 0.22 0.76 0.020 0.015 1.11 0.04 0.023 0.02 0.002 0.0132 0.0006 1t-VIM 23 0.18 0.24 0.80 0.021 0.014 1.12 0.05 0.029 0.01 0.001 0.0152 0.0012 1t-VIM 24 0.19 0.25 0.81 0.020 0.015 1.05 0.06 0.028 0.01 0.001 0.0145 0.0018 1t-VIM 25 0.21 0.17 0.78 0.014 0.014 1.21 0.08 0.026 0.16 0.002 0.0121 0.0007 Nb: 0.02 Electric furnace 26 0.18 0.18 0.75 0.018 0.017 1.22 0.05 0.025 0.15 0.003 0.0129 0.0008 Nb: 0.02 1t-VIM 27 0.20 0.17 0.78 0.015 0.016 1.09 0.05 0.029 0.15 0.001 0.0112 0.0010 Nb: 0.03 1t-VIM 28 0.21 0.19 0.79 0.018 0.014 1.12 0.05 0.022 0.15 0.002 0.0125 0.0014 Nb: 0.04 1t-VIM 29 Ti-added steels 0.19 0.12 0.43 0.019 0.024 1.16 0.06 0.021 0.02 0.099 0.0135 0.0010 B: 0.0011 Electric (chemical furnace 30 cooutsidemposition 0.18 0.09 0.41 0.018 0.020 1.21 0.04 0.020 0.01 0.112 0.0128 0.0010 B: 0.0012 1t-VIM 31 scope of claim) 0.18 0.10 0.41 0.015 0.021 1.09 0.04 0.025 0.01 0.100 0.0133 0.0012 B: 0.0016 1t-VIM 32 0.17 0.11 0.42 0.019 0.022 1.02 0.04 0.029 0.01 0.088 0.0114 0.0012 B: 0.0014 1t-VIM 33 0.21 0.15 0.82 0.015 0.022 1.06 0.06 0.021 0.02 0.234 0.0060 0.0011 Electric furnace 34 0.19 0.14 0.83 0.014 0.018 1.11 0.10 0.018 0.01 0.222 0.0072 0.0009 1t-VIM 35 0.18 0.12 0.81 0.013 0.023 1.00 0.09 0.017 0.01 0.202 0.0053 0.0012 1t-VIM 36 0.18 0.14 0.78 0.014 0.022 1.12 0.08 0.019 0.01 0.238 0.0064 0.0011 1t-VIM * In Table 2, the values in the underlined part are outside the scope of claim.

The √ (area max) value as the maximum size of TiN or TiCN was estimated from steel bars having a size of 60 mmφ in a measurement area of 30,000 mm² for the middle and peripheral parts of the steel bars by an extreme value statistical method. Further, test pieces for a carbide tool wear test by turning and test pieces for a drill life test were prepared from 60-mmφ steel bars and were evaluated for machinability. Conditions for the carbide tool wear test by turning are shown in Table 3, and conditions for the drill life test are shown in Table 4. On the other hand, test pieces for a Charpy impact test (rectangular shape: 10 mm×55 mm-2 mm 10 RC notch) and test pieces for a three-point bending test (rectangular shape: 10 mm×70 mm-2 mm V notch) were prepared from 32-mmφ steel bars and were tested. For the test pieces for the Charpy impact test and the test pieces for the three-point bending test, the 32 mmφ normalized material was subjected to turning to a size of 30 mmφ and was then subjected to drawing with a reduction in area of 50%. The steel bar was divided into test pieces. The test pieces were then carburized and quenched at 950° C., were tempered at 180° C., and were evaluated. TABLE 3 Conditions for Item carbide tool wear test by turning Tool Material: ST20E Cutting edge: 0.4R Machining speed (m/min) 150 Cut (mm) 0.5 Feed (mm/rev) 0.25 Machining oil None Evaluation method Abrasion loss of flank in chip after turning for 10 min

TABLE 4 Item Conditions for drill life test Tool Material: SKH55 5 mmφ-JIS straight Machining speed (m/min) 30 Feed (mm/rev) 0.30 Hole depth (mm) 15 (blind hole) Machining oil Water soluble Evaluation method Number of holes formed until hole formation became impossible

The test results for the above various cast steel billet sizes and steel ingots sizes are shown in Table 5. In Table 5, the cast steel billet size and the steel ingot size are shown in diameter for round semi-finished steel products and round steel ingots and in short side length for rectangular cast steel billets and steel ingots. TABLE 5 Size of cast √ area Abrasion loss Number of Load for crack steel billet max of TiN of carbide holes Impact initiation in and steel or TiCN turned formed by strength three-point No. Classification ingot (cm) (μm) chip (mm) drilling (J/cm²) bending test (N) 1 Ti-added steels 38 61 0.18 18 22 11000 2 (chemical 16 26 0.09 74 23 11100 3 composition 30 53 0.14 24 23 11300 4 falling within 23 34 0.09 69 25 11200 5 scope of claim) 38 59 0.18 18 21 10500 6 16 25 0.08 74 22 10100 7 30 53 0.14 24 21 10800 8 23 36 0.09 69 22 10900 9 38 58 0.18 17 19 11100 10 16 28 0.08 69 21 11100 11 30 53 0.14 24 20 11300 12 23 35 0.09 71 21 11100 13 38 55 0.18 19 20 11400 14 16 23 0.07 78 22 11300 15 30 53 0.14 28 20 11600 16 23 39 0.10 68 21 11200 17 38 59 0.18 21 23 12800 18 16 21 0.07 72 23 12900 19 30 53 0.14 25 22 12800 20 23 32 0.10 65 25 13200 21 Ti-free steels 38 49 0.12 64 16 7800 22 (comparative 16 25 0.11 74 16 8700 23 steels) 30 52 0.11 54 17 8300 24 23 34 0.11 68 18 8500 25 38 45 0.12 54 16 8200 26 16 26 0.12 65 18 8900 27 30 44 0.11 71 16 9000 28 23 25 0.11 58 17 9200 29 Ti-added steels 38 83 0.28 20 19 11300 30 (chemical 16 56 0.16 29 21 11600 31 composition 30 73 0.24 18 20 11100 32 outside 23 54 0.19 26 21 11300 33 scope of claim) 38 84 0.30 13 18 11200 34 16 57 0.17 23 20 11400 35 30 76 0.26 16 20 11200 36 23 58 0.20 25 19 11000 * Diameter for round cast steel billets and round steel ingots, and short side length for the other cast steel billets and steel ingots.

In Table 5, for test pieces indicated by odd numbers, the diameters in the case of the round type and the short side lengths in the case of the rectangular type exceed 23 cm for the cast steel billet and steel ingot sizes. On the other hand, for test pieces indicated by even numbers, the diameters in the case of the round type and the short side lengths in the case of the rectangular type are not more than 23 cm the for the cast steel billet and steel ingot sizes. As can be seen in Table 5 and FIG. 1, among titanium-added case hardening steel Nos. 1 to 20 having chemical compositions of the present invention, 10 steels indicated by even numbers had small diameters or short side lengths of not more than 23 cm for cast steel billet and steel ingot sizes. In this case, the √ (area max) value as the maximum size of TiN or TiCN was not more than 39 μm which was smaller than that of the 10 steels indicated by the odd number.

Further, from Table 5 and FIG. 1, when comparison is made among the steels having the same size for cast steel billet and steel ingot, it is apparent that the √ (area max) values as the maximum size of TiN or TiCN in the titanium-added case hardening steels comprising constituents falling within the scope of the present invention indicated by Nos. 1 to 20 in Table 5 are smaller than those of the titanium-added steels comprising constituents outside the scope of the present invention indicated by Nos. 29 to 36 in Table 5.

Further, from Table 5 and FIG. 2, it is apparent that, among the titanium-added steels comprising constituents falling within the scope of the present invention indicated by Nos. 1 to 20 in Table 5, the 10 steels according to the present invention indicated by even numbers, which had small diameters or short side lengths of not more than 23 cm for cast steel billet and steel ingot sizes, had a tendency that the abrasion loss of the chip in the carbide tool turning test was reduced with reducing the √ (area max) value as the maximum size of TiN or TiCN, and had smaller √ (area max) value than the steels indicated by odd numbers having a diameter or short side length of more than 23 cm for cast steel billet and steel ingot size.

From Table 5 and FIG. 3, it is apparent that, among the titanium-added steels according to the present invention indicated by Nos. 1 to 20 in Table 5, the steels indicated by even numbers, which had small √ (area max) values as the maximum size of TiN or TiCN and had diameters or short side lengths of not more than 23 cm for cast steel billet and steel ingot sizes, are larger in number of holes formed by drilling than the steels indicated by odd numbers.

For titanium-added steels comprising constituents falling within the scope of the present invention indicated by Nos. 1 to 20 in Table 5, grain coarsening was not observed, and, accordingly, the impact strength and the crack initiation load in the three-point bending test are higher than those of the titanium-free steels as comparative steels indicated by Nos. 21 to 28 in Table 5, demonstrating the superiority of the titanium-added steels according to the present invention. 

1. A process for producing a cast steel billet or steel ingot of a titanium (Ti)-added case hardening steel, said process comprising the steps of: melting a steel comprising by weight carbon (C): 0.10 to 0.30%, titanium (Ti): 0.05 to 0.20%, and nitrogen (N): not more than 0.0100%; and casting the molten steel into a round cast steel billet or steel ingot having a diameter of not more than 23 cm.
 2. The process according to claim 1, wherein said steel comprises by weight carbon (C): 0.10 to 0.30%, silicon (Si): 0.03 to 1.00%, manganese (Mn): 0.20 to 1.00%, phosphorus (P): not more than 0.025%, sulfur (S): 0.001 to 0.030%, nitrogen (N): not more than 0.100%, titanium (Ti): 0.05 to 0.20%, aluminum (Al): 0.003 to 0.050%, oxygen (O): not more than 0.0030%, one or more of chromium (Cr): 0.15 to 2.00%, nickel (Ni): 0.10 to 2.00%, molybdenum (Mo) 0.03 to 0.30%, boron (B): 0.0001 to 0.0020%, vanadium (V): 0.01 to 0.20%, and niobium (Nb): 0.01 to 0.15%, and the balance iron (Fe) and unavoidable impurities.
 3. A process for producing a cast steel billet or steel ingot of a titanium (Ti)-added case hardening steel, said process comprising the steps of: melting a steel comprising by weight carbon (C): 0.10 to 0.30%, titanium (Ti): 0.05 to 0.20%, and nitrogen (N): not more than 0.0100%; and casting the molten steel into a rectangular cast steel billet or steel ingot having a short side length of not more than 23 cm.
 4. The process according to claim 3, wherein said steel comprises by weight carbon (C): 0.10 to 0.30%, silicon (Si): 0.03 to 1.00%, manganese (Mn): 0.20 to 1.00%, phosphorus (P): not more than 0.025%, sulfur (S): 0.001 to 0.030%, nitrogen (N): not more than 0.0100%, titanium (Ti): 0.05 to 0.20%, aluminum (Al): 0.003 to 0.050%, oxygen (O): not more than 0.0030%, one or more of chromium (Cr): 0.15 to 2.00%, nickel (Ni): 0.10 to 2.00%, molybdenum (Mo) 0.03 to 0.30%, boron (B): 0.0001 to 0.0020%, vanadium (V): 0.01 to 0.20%, and niobium (Nb): 0.01 to 0.15%, and the balance iron (Fe) and unavoidable impurities. 